docs: def ser(ctx, source, output_format):
      """SERIALIZE: tranform an RDF graph to a format of choice.
      """
      verbose = ctx.obj['VERBOSE']
      sTime = ctx.obj['STIME']
      print_opts = {
          'labels': verbose,
      }
      output_format = output_format
      VALID_FORMATS = ['xml', 'n3', 'turtle', 'nt', 'pretty-xml', "json-ld"]
      if not source:
          click.echo(ctx.get_help())
      else:
          if output_format not in VALID_FORMATS:
              click.secho(
                  "Not a valid format - must be one of: 'xml', 'n3', 'turtle', 'nt', 'pretty-xml', 'json-ld'.",
                  fg='red')
              return
          else:
              action_serialize(source, output_format, verbose)
              eTime = time.time()
              tTime = eTime - sTime
              printDebug(
                  "\n-----------\n" + "Serialized <%s> to '%s'" %
                  (" ".join([x for x in source]), output_format), "comment")
              printDebug("Time:	   %0.2fs" % tTime, "comment")